If you apply the following style to any Plantuml exported SVG:
filter: invert(100%) !important;
-webkit-filter: invert(100%) !important;
you will notice that the canvas surface (the one the diagram's title is located on) will not be inverted (it is stayed white with white inverted title). Everything else in the SVG gets inverted correctly.
I played a bit with the svg and style and could not get this fixed.
Could you check if this is an issue of the exported svg or is there a workaround?
Here is what you see before and after the style applied: